The U.S. government is closed. And if it doesn't reopen by Monday, Americans expecting refunds or waiting to hear back from the IRS on an existing audit or other tax matter may be out of luck.

The government shut down early Saturday after Congress failed to agree on a funding bill. That triggered automatic furloughs for government employees deemed "non-essential." That means only a portion of each federal agency's employees will continue to work through the shutdown.

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The IRS is keeping 35,076 employees on the job -- that's about 43.5% of its total workforce. If it were any other time of the year, more IRS employees would have been furloughed.

But this is tax filing season. That's defined by the IRS as between January 1 and April 30, 2018, according to the agency's 2018 Lapsed Appropriations Contingency Plan.

The contingency plan gives examples of the IRS functions that will continue throughout the shutdown. They include processing electronic returns, testing upcoming filing year programs, and computer operations to prevent the loss of data.

Unless the shutdown drags on for weeks, the refund you're owed for 2017 likely won't be affected. Why? The IRS already announced it wouldn't start accepting 2017 tax returns until January 29.

So in the short term, the people most likely to be affected by the halt on refunds are those who are owed them for earlier tax years.

The Treasury and IRS have the option to reassess how many people they need working and what functions may be performed.

If this shutdown lasts more than five days, however, there could be changes. Treasury, which issues the IRS contingency plan, has the option to change its mind on whether it will keep withholding returns.

There's also a chance the House and Senate will reopen the government by Monday. Both chambers are working over the weekend in the hopes of putting a stop gap measure in place.

But there are no guarantees, and a shutdown is the last thing the IRS needs right now.

The agency is in the midst of implementing the most significant tax code overhaul in more than 30 years.

Even before a shutdown loomed, National Taxpayer Advocate Nina Olson warned that the IRS needs more funding and staffing in order to adequately implement the new tax law.

That includes a need for more staff to help answer the questions millions of Americans will inevitably have.

Even before the new tax law was signed in December, the IRS was projecting that its representatives would only be able to handle four out of every 10 taxpayer calls in fiscal year 2018.